Prostanoids are a family of eicosanoids that comprise prostaglandins (PGs), prostacyclins (PGIs), and thromboxanes (Txs). Their receptors belong to the G-protein coupled receptor (GPCR) superfamily of receptors and may be grouped into five classes, namely, prostaglandin D (DP), prostaglandin E (EP), prostaglandin F (FP), prostaglandin I (IP), and Thromboxane A (TP) based on their sensitivity to five naturally occurring prostanoids, PGD2, PGE2, PGF2[alpha], PGI2, and TxA2, respectively (Coleman, R. A., 2000).
Prostaglandins are small potent inflammatory mediators that are generated by the release of arachidonic acid (AA) from the membrane phospholipids. Subsequently, cyclooxigenase and prostaglandin synthase enzymes metabolize AA to prostaglandins that play pivotal roles in the modulation of physiological systems, such as CNS, and the inflammatory and immune responses.
Prostaglandins contribute to the sensitization of peripheral and central nociceptive neurons during peripheral inflammation (Dirig and Yaksh, 1999) and play an important role in the pathogenesis of neuropathic pain following nerve injury (Syriatowicz et al 1999; Samad et al, 2002; Ma and Eisenach, 2003).
Prostaglandin E2 (PGE2) is considered to be the dominant pro-nociceptive prostanoid. Guay and colleagues, analyzing the concentrations of different prostaglandins in the cerebrospinal fluid, found that PGE2 was the most prevalent prostanoid and exhibited the highest increase after peripheral carrageenan-induced inflammation (Guay et al., 2004). PGE2 is generated in most cells in response to mechanical, thermal or chemical injury and inflammatory insult, resulting in sensitization or direct activation of nearby sensory nerve endings. Its production requires the activity of at least one of the two cyclooxygenase isoforms, COX-1 constitutively expressed or COX-2 which is inducible and particularly relevant for inflammation-induced PGE2 formation. Therefore, non-selective inhibitors of COX-1 and COX-2, and selective COX-2 inhibitors provide good pain relief. However, the long-term use is associated with gastrointestinal or cardiovascular side effects, respectively.
Downstream components of the inflammatory cascade could be an alternative approach for the treatment of the PGE2 associated pain. PGE2 binds to four different G-protein coupled receptors named EP1, EP2, EP3 and EP4 (Narumiya et al., 1999).
Studies employing antagonists suggest that blocking EP1, EP2, EP3 or EP4 receptors may reduce certain types of pain (Oka et al. 1997; Lin et al, 2006). Among these PGE2 receptors, most of the drug discovery studies have focused on modulating EP4 receptor. EP4 receptor has been associated in various models of immune response, inflammation, hypoxia, organ damage, autoimmunity, bone catabolism and transplantation (M. Zimecki, 2012), revealing therapeutic utility of application of either agonist or antagonist of EP4 receptor.
EP4 receptor couples mainly to Gs and mediates transient increase in intracellular cAMP concentration. In turn, cAMP activates protein kinase A (PKA), which then phosphorilates downstream effector proteins, in particular cAMP response element-binding protein (CREB). Furthermore, an EP4 receptor-associated protein (EPRAP) which binds to the unique long carboxyl terminal cytoplasmatic domain of EP4 receptor has been described to participate in anti-inflammatory signalling (Takayama, K. et al. 2006). In addition, EP4 receptors activate the phosphatidylinositol 3-kinase (PI3K) signalling pathway (Fujino et al., 2003).
Rheumatoid arthritis (RA) is a chronic inflammatory disorder leading to bone and cartilage destruction. A substantial body of evidence suggests that prostaglandin E2 (PGE2) contributes to the pathogenesis of RA, and nonsteroidal anti-inflammatory drugs, inhibitors of the synthesis of PGE2 and other prostanoids, continue to be used in the treatment of this disease.
McCoy and colleagues examined mice lacking each of the four known PGE2 (EP) receptors after generation of collagen antibody-induced arthritis, an animal model of RA. Homozygous deletion of the EP1, EP2, or EP3 receptors did not affect the development of arthritis, whereas EP4 receptor-deficient mice showed decreased incidence and severity of disease. These animals also showed reduced inflammation as assessed by circulating IL-6 and serum amyloid A levels. Joint histopathology of EP4−/− animals revealed reduced bone destruction, proteoglycan loss, and type II collagen breakdown in cartilage compared with EP4+/+ mice. Furthermore, liver and macrophages isolated from EP4−/− animals produced significantly less IL-1β and IL-6 than control samples. Thus, PGE2 contributes to disease progression at least in part by binding to the EP4 receptor. Antagonists of this receptor might therefore provide novel agents for the treatment of RA. (McCoy et al. 2002)
Recent studies involving parenteral administration of several EP4 antagonists (AH-23848, CJ-023423, CJ-042794, MF-498, ONO-AE3-208) have clearly demonstrated a major involvement of EP4 receptors in small-animal models of inflammation. Joint pain, mechanical and thermal hyperalgesia and edema were markedly suppressed, often equivalent to the efficacy of selective COX-2 inhibitors such as rofecoxib. (Jones et al, 2009). These are more evidences that EP4 antagonists might provide novel agents for the treatment of rheumatoid arthritis and osteoarthritis.
Two distinct helper T (TH) subsets, TH1 and TH17, mediate tissue damage and inflammation in animal models of various immune diseases such as multiple sclerosis, rheumatoid arthritis, inflammatory bowel diseases, psoriasis and other allergic skin disorders. These experimental findings, and the implication of these TH subsets in human diseases, suggest the need for pharmacological measures to manipulate these TH subsets. Yao C. and colleagues showed that prostaglandin E2 (PGE2) acting on its receptor EP4 on T cells and dendritic cells not only facilitates TH1 cell differentiation but also amplifies interleukin-23-mediated TH17 cell expansion in vitro. Administration of an EP4-selective antagonist in vivo decreases accumulation of both TH1 and TH17 cells in regional lymph nodes and suppresses the disease progression in mice subjected to experimental autoimmune encephalomyelitis, contact hypersensitivity or colitis model (Yao C., 2009 and 2013) and different remathoid arthritis models (Chen Q., et al. 2010). Thus, PGE2-EP4 signaling promotes immune inflammation through TH1 differentiation and TH17 expansion, and EP4 antagonism is proposed as a promising drug target for immunomodulation and may be therapeutically useful for various immune diseases (Yao C. et al. 2009).
Significant cross-talk can occur between the cannonical signalling pathways described above and several additional pathways can be activated in some cells. One of the most important of these is the recent demonstration that the EP1, EP2 and EP4 receptors can transactivate the epidermal growth factor receptor (EGFR) which is implicated in proliferation, invasion, resistance to apoptosis, angiogenesis, and metastasis, all of which are associated with tumor development (Wu, W. et al. 2010). Angiogenesis is also closely linked with clinical manifestations of non-noplastic diseases such as some autoimmune diseases (e.g. psoriasis, reumathoid arthritis, . . . ), age-related macular degeneration and atherosclerosis (Folkman J., 2007, Heidenreich R., 2009).
Accumulating evidence indicates that elevated levels of prostaglandin E2 (PGE2) can increase intestinal epithelial cell proliferation, and thus play a role in colorectal tumorigenesis. PGE2 exerts its effects through four G-protein-coupled PGE receptor (EP) subtypes, named the EP1, EP2, EP3, and EP4. Increased phosphorylation of extracellular regulated kinases (ERK1/2) is required for PGE2 to stimulate cell proliferation of human colon cancer cells. Cherukuri and colleagues provide evidence that L-161,982, a selective EP4 receptor antagonist, completely blocks PGE2-induced ERK phosphorylation and cell proliferation of HCA-7 cells. They concluded that egr-1 is a target gene of PGE2 in HCA-7 cells and is regulated via the newly identified EP4/ERK/CREB pathway (Cherukuri et al., 2007). These results support the notion that antagonizing EP4 receptors may provide a novel therapeutic approach to the treatment of colon cancer.
EP4 receptor antagonists may have therapeutic utility in the treatment of migraine since it has been observed that EP4 antagonists block PGE2-induced relaxation of human-isolated middle cerebral artery (Davis et al., 2004; Maubach et al., 2009) and the picture has been enlarged to include the interaction of endogenous PGE2 with calcitonin gene-related peptide release from trigeminal nerves (Maubach et al., 2009).
Chuang and colleagues found that MF191, a selective EP4 receptor antagonist, may have effects on the bladder urothelium and inflammatory cells and suppress CYP- or PGE 2-induced bladder overactivity (Chuang et al, 2012). EP4 receptor antagonists may be useful for the treatment of overactive bladder.
Additional therapeutic applications for EP4 antagonists are modulation of the cough reflex (Maher et al 2010), treatment for endometriosis in women (Lee et al 2010) and Alzheimer's disease (Wei et al, 2010).
Based on the above mentioned results coming from animal and human studies, EP4 receptor has been identified as a selective target for the development of new potential therapies for the treatment of those disorders where PGE2 action is involved. In view of the potential therapeutic applications of agonists and antagonists of the EP4 receptor, a great effort is being directed to find selective ligands. Despite intense research efforts in this area, very few compounds with selective EP4 activity have been reported.
There is thus still a need to find compounds having pharmacological activity towards the EP4 receptor, being both effective and selective, having good “druggability” properties, i.e. good pharmaceutical properties related to administration, distribution, metabolism and excretion and showing a good toxicological profile.
The present invention hereby provide some novel compounds complying with the above mentioned properties.